Do mammals lay eggs?
Could a rabbit really lay eggs? Rabbits are mammals, and most mammals do not lay eggs. Mammals usually give birth to live offspring.
You probably realize that cats, dogs, people, and other mammals don't typically lay eggs. There are exceptions, however. Egg-laying is possible among the monotremes, mammals with birdlike and reptilian characteristics. The egg-laying monotremes, such as echidnas (Figure below) and platypuses (Figure below), use one opening, the cloaca, to urinate, release waste, and reproduce; birds do the same. They lay leathery eggs, similar to those of lizards, turtles, and crocodilians. Monotremes feed their young by “sweating” milk from patches on their bellies; they lack the nipples present on other mammals.
The echidna (left) is a member of the monotremes, the most primitive order of mammals. Another monotreme, the platypus (right), like other mammals in this order, lays eggs and has a single opening for the urinary, genital, and digestive organs.
All other mammals give birth to live young and belong to one of two different categories:
Marsupial: Animals in which the embryo is born at an immature stage. Development must be completed outside the mother's body. Most female marsupials have an abdominal pouch or skin fold where there are mammary glands; it is a place for raising the young (Figure below).
Placental: female placentals have a placenta, a spongy structure that passes oxygen, nutrients, and other useful substances from the mother to the fetus. It also passes carbon dioxide and other wastes from the fetus to the mother. The placenta allows the fetus to grow for a long time within the mother.
A marsupial mammal, this eastern gray kangaroo has a joey (young kangaroo) in its abdominal pouch.
Some mammals are alone until a female can become pregnant. Others form social groups with big differences between sexes, such as size differences, a trait called sexual dimorphism. Dominant males are those that are largest or best-armed. These males usually have an advantage in mating. They may also keep other males from mating with females within a group. This is seen in elephant seals (Figure below).
A mating system with a group of many females and one male, as seen in the seal species.
cloaca: Single exit and entrance for sperm, eggs, and waste.
marsupial: Mammal in which the young are born in an immature state and continue development in the pouch.
monotreme: Mammal with some birdlike and reptilian characteristics, such as laying eggs.
placenta: Spongy structure that passes oxygen, nutrients, and other useful substances from the mother to the fetus.
placental: Mammal with an organ that feeds the fetus and removes waste products.
sexual dimorphism: Distinct difference in appearance between the males and females.
- Monotremes can lay eggs, but most mammals give birth to live young.
- Mammals can be marsupial, where the embryo is born at an immature stage and develops in the pouch.
- Mammals can be placental, where substances are passed from the mother to the fetus so that it can stay longer in the womb.
Use the resources below to answer the questions that follow.
- What are some examples of marsupials?
- How do marsupial and placental mammals differ?
- How does parental care in marsupial mammals differ from parental care in placental mammals?
- Why do marsupials lick themselves during birth?
- How do scientists think marsupial babies find the pouch?
- What are some of the purposes of the placenta?
- What does the placenta allow to pass to the embryo?
- What does the placenta keep from the embryo?
- What is the function of the umbilical cord?
- How are monotremes more like reptiles and birds than other mammals?
- What's the difference between marsupial and placental animals?